Molecular interactions form the foundation of biology and chemistry. They are central to life itself and determine catalytic activity, cellular function, and therapeutic efficacy. The vast majority of diagnostic procedures depend on some type of specific molecular interaction. Therefore, the ability to perform pure liquid-phase molecular binding analysis at high sensitivity, without modifying the interacting species, and at physiological concentrations would be revolutionary. Yet, the tools available to quantify these interactions have limitations. Traditional methods such as the sucrose gradient technique or isothermal titrimetric calorimetry are laborious and require substantial quantities of sample to perform an assay. Fluorescence and radioactive methods are sensitive, but rely on the incorporation of signaling labels to enable detection, slowing the assay and increasing its cost. Numerous techniques, particularly the label-free methods, require surface immobilization of one of the interacting moieties putting the species in a non-native environment. Labels and tethers can be benign, but often alter the interacting molecules and can lead to a biased result. Recently my group and others demonstrated that back-scattering interferometry (BSI) can be used in the academic laboratory to perform molecular interaction determinations label-free and in free-solution, with sensitivity that allows assays on small quantities of sample, at physiologically relevant concentrations. BSI is a universal sensing method that only requires the product of a reaction to refract or interact with light differently than the participating species, therefore has the potential to be widely applicable for general use as a Molecular Interaction Photometer (MIP). BSI is a prime candidate to become an MIP because it has a simple and inexpensive optical train comprised of a He-Ne laser, a microfluidic channel, and a position sensor allowing minute refractive index changes to be monitored. Measurements are made within a microfluidic channel formed in glass, fused silica, or plastic and at physiologically relevant concentrations in complex matrices such as serum or with native membrane- proteins. Yet the current academic embodiment of BSI is not commercially viable. Tedious alignment methods, immature transduction schemes, poorly refined sample handling and introduction methods, and performance limitations due to environmental noise sensitivity all impede the wide dissemination and adoption of BSI in the life science and drug discovery communities. Under Phase I of this STTR grant we met our milestones demonstrating a two-channel BSI instrument with a pipette-driven injection method and a fringe detection algorithm that simplified fringe selection and alignment. Here we propose to build on these results, while capitalizing on two new innovations to transform our academic laboratory BSI into the MIP instrument we call NanoBIND. Under this STTR Phase II, we propose the completion of four aims to refine BSI through research development and technology transfer, facilitate commercialization by Molecular Sensing Inc. and allow the subsequent broad dissemination in the research community. In Aim 1 we further simplify the optical train, while retaining the advantages of performing a simultaneous sample-reference determination. Aim 2 implements a sample introduction method that is user friendly, minimizes the potential for contamination and constrains volumes to <1?L. An improved algorithm enhances sensitivity, enables electronic fringe selection and alignment and addresses non-specific interactions (at cell wall) to improve assay reproducibility in Aim 3. And in Aim 4 ?- prototype version BSI systems will be constructed and used, external to Vanderbilt and MSI, to demonstrate that BSI gives meaningful and quantitative binding affinity values (from ?M to pM) and that it can be used to screen for molecular interactions in complex matrices such as serum and cell-free media, as well as DMSO. Upon completion of these Aims, three identical prototypes will have been constructed and evaluated for field utility. Feedback from these laboratories and users will provide the formal framework for refining the design under Phase III commercial deployment.